The present invention relates to powder metallurgy (P/M) stainless steel powders and compacts therefrom, and more particularly to improving the corrosion resistance of such powders and compacts.
Heretofore, poor corrosion resistance of such compacts has been attributed mainly to the porosity found within the compacts, thus most techniques for overcoming corrosion problems have been aimed at closing the porosity. Prior techniques aimed at minimizing the surface porosity of the compacts made from such stainless steel powders include mechanical closure treatment, plastic impregnation, surface coatings, or passivation treatments. Each of these techniques has some limitation as to its effectiveness in addition to raising the cost of the final support product. Other proposals aimed at improving the corrosion resistance of stainless steel powder compacts concentrate on compacting and sintering parameters. These proposals generally state that the sintering conditions and sintering atmosphere have a marked influence on the corrosion properties of the powder compacts; however, most of the experimental results reported in these proposals are inconsistent. Japanese Tokkai No. 35708 (1977) suggests that a small proportion of tin can be added to some stainless steel powder compacts to enhance the corrosion resistance. However, great care must be taken in preparing compacts according to the Japanese specification because the carbon content of the compacts must be maintained at less than 0.05%. Higher carbon levels in the parent alloy and carbon pickup during processing must be avoided according to the Japanese specification in order to maintain corrosion resistance of the compacts made.
In accord with the present invention, it has now been found that tin containing stainless steel powder and compacts thereof having greater than 0.05% carbon and excellent corrosion resistance can be produced if an additional modifier metal selected from the group consisting of niobium, titanium, tantalum, molybdenum, and the like is alloyed with the parent alloy during the powder formation.
It is an object, therefore, of the present invention to add a modifier metal selected from the group consisting of niobium, titanium, tantalum, molybdenum, and the like and mixtures thereof to a tin containing stainless steel powder or compact thereof having greater than 0.05% carbon. A further object and advantage of the present invention is to produce the stainless steel powder or compact thereof having superior corrosion resistance when compared to similar high carbon powders and compacts which do not contain the modifier metal. A still further object and advantage of the present invention is that corrosion resistant stainless steel powder compacts can be produced without the heretofore required rigid controls on carbon content and pickup. Still further objects and advantages will become more evident from the detailed description of the invention.